1. Field of the Invention
The present invention relates to a non-aqueous liquid electrolyte cell which uses a metal oxide as an active material of a positive electrode.
2. Prior Art
Non-aqueous liquid electrolyte cells are widely used as power sources of electric clocks or watches or memory backup power sources, since they can be used in a wide temperature range and have long-term reliability. However, the non-aqueous liquid electrolytes have a low ion conductivity of about one fortieth of that of aqueous electrolytes, since they comprise organic solvents as the solvents of liquid electrolytes. Therefore, the non-aqueous liquid electrolyte cells are designed so that the amount of the liquid electrolyte in the cells is larger than that in alkaline cells comprising an alkaline solution as a liquid electrolyte, since the discharge capacity of the former cell decreases when the amount of the liquid electrolyte in the non-aqueous liquid electrolyte cells is the same as that in the alkaline cells. In general, the conventional non-aqueous liquid electrolyte cells contain about 1.4 times volume of the liquid electrolyte in relation to the volume of a metal oxide as the active material of the positive electrode.
To increase the capacity of the cells, it is desired to increase the charging amount of active material as much as possible in addition to the increase of the conductivity of electrodes. However, the amounts of the active material can be increased in the limit of the volume of the cell, since the cell size is fixed. Therefore, it is necessary to effectively utilize the internal volume of the cell.
Thus, it may be contemplated to increase the amounts of the active material by the decrease of the volumes occupied by constituent elements other than the active materials. For example, it is proposed to use a separator made of a thin microporous film having a uniform thickness or to decrease the wall thickness of a cell can (JP-A-60-23954, etc.) However, a sufficiently high capacity has not been achieved. In particular, the reduction of the thickness of exterior parts such as the cell can is reaching its limit from the viewpoint of the liquid leakage.
It is proposed to decrease the content of constituent components (e.g. conducting aids, binders, etc.) of the positive electrode other than the active material to increase the content of the active material in the positive electrode (JP-A-7-272714, etc.) However, it is a matter of degree, and this approach cannot present fundamental solutions.
As an alternative method to decrease the volumes of cell elements, it is contemplated to decrease the amount of the non-aqueous liquid electrolyte which is charged in the cell in a relatively large amount. However, when the amount of the liquid electrolyte is decreased, a designed voltage can be achieved in the initial discharge period, but the amount of the liquid electrolyte becomes insufficient in the separator or on the surface of the active material of the positive electrode in the course of discharging for the following reasons, and thus the liquid connection between the positive electrode and the negative electrode is interrupted so that the designed discharge capacity is not achieved.
That is, when the cell is discharged, alkaline metal ions move from the active material of the negative electrode to the positive electrode and are intercalated in the metal oxide as the active material of the positive electrode. Therefore, the active material of the positive electrode expands to induce the increase of the surface area or the increase of the volume of vacancies in the positive electrode. To allow the discharge reaction of the active material to proceed smoothly, the liquid electrolyte should be present on the surface of the active material in an amount corresponding to the surface area of the active material. However, the cell containing the limited amount of the liquid electrolyte cannot cope with the above increase of the surface area of the active material. Furthermore, the amount of the liquid electrolyte maintained in the separator becomes insufficient, since the increase of the volume of the vacancies in the positive electrode causes the shift of the liquid electrolyte from the separator to the positive electrode. Accordingly, when the amount of the liquid electrolyte is decreased and the higher amount of the active material is charged in the cell, the amount of the liquid electrolyte becomes insufficient in the last period of discharge so that the active material cannot be utilized, and thus the discharge capacity decreases.
In particular, the above decrease of the discharge capacity is significant in the case of discharging at low temperature. To increase the discharging characteristics of the cells at low temperature, usually, positive electrode active materials having a smaller particle size than conventional one (about 15 to 30 xcexcm) are used. However, such active materials have a large surface area, and therefore cannot solve the above problems but they may deteriorate the characteristics.
One object of the present invention is to provide a non-aqueous liquid electrolyte cell having a high capacity and good characteristics even at low temperature.
Accordingly, the present invention provides a non-aqueous liquid electrolyte cell comprising a positive electrode which comprises a metal oxide as an active material, a negative electrode, and a non-aqueous liquid electrolyte, wherein said metal oxide has an average particle size of 40 to 150 xcexcm. and the volume of said liquid electrolyte is 0.9 to 1.25 times as large as the volume of said metal oxide.